This invention relates to a light measuring apparatus to be used in an automatic exposure-control apparatus or for an exposure-meter in cameras which computes exposure information from inputs such as photographic objective brightness, film sensitivity, aperture value, etc.
In the conventional automatic exposure-control circuit for a single-lens reflex camera employing a TTL (through-the-lens) light measuring system, since the objective brightness ranges over a considerably wide area, a voltage to be stored in a storage capacitor as an electric signal representing the measured brightness varies in proportion to the logarithm of brightness; the so-called logarithm-compression process is employed.
The exposure time is determined by information representative of objective brightness, as well as by information representative of film sensitivity and aperture value. In this process the film sensitivity and the aperture value vary exponentially, such as twofold, fourfold, eightfold, and so on. In order to apply such quantities, together with the information of logarithmically compressed objective brightness to the control circuit, the information representative of film sensitivity and aperture value is also converted into logarithmic quantities. This type of computation is carried out in an APEX system.
The conventionally employed circuit for controlling the exposure time in accordance with information representative of logarithmically compressed objective brightness, film sensitivity and aperture value has been constituted as shown in FIG. 1, wherein Vs indicates a voltage source, for instance, a photodiode or solar battery, to produce a voltage Vs proportional to the objective brightness. A luminance value Bv which responds to an objective brightness in the APEX system is represented by the output voltage Vs of this voltage source Vs. DA1 indicates a differential amplifier, and to its non-inversion input terminal "+" said information of objective brightness is imparted. Q1 is a transistor for imparting a negative inversion to the differential amplifier DA1, and the collector of the transistor is connected to an inversion input terminal "-" of the differential amplifier DA1.
VR indicates a variable resistor for setting a resistance value in accordance with the film sensitivity and the aperture value, and its resistance value is changed exponentially for each change of one step in the film sensitivity or the aperture value. This variable resistor VR and a diode D2 are connected in series between a power source terminal V and ground (the power source between terminal V and ground being omitted in the drawing). A transmission Q2 and a resistor R1 constitute an emitter-follower circuit, which produces a voltage appearing at the diode D2 at the emitter of transistor Q2. D1 indicates a diode connected across the collector of the transistor Q1 and the emitter of the transistor Q2.
The following is an explanation of the function of prior-art circuit shown in FIG. 1. The differential amplifier DA1 receives a negative feedback from the collector of the transistor Q1. As a result, the voltage V1 between the collector of the transistor Q1 and ground becomes equal to the voltage Vs which is proportional to the information Bv of the objective brightness. If the resistance value of the variable resistor VR is set in accordance with the film sensitivity and the aperture value, a current corresponding to this resistance value flows into the diode D2. Across the diode D2, a voltage proportional to the logarithm of this current appears, and this voltage is impressed as an input signal to the emitter-follower circuit which is constituted by the transistor Q2 and the resistor R1.
The variable resistor VR is designed to have a characteristic such that its resistance value varies exponentially, and the voltage appearing across the diode D2 is proportional to the logarithm of the current flowing therein. Therefore, the input voltage of the emitter-follower circuit varies linearly relative to each step of variation in the film sensitivity and the aperture value, and each step corresponds to each variation of an equiangular rotation of the variable resistor VR.
Due to the above-mentioned functions, a voltage Vr which varies linearly for each stepwise difference of the film sensitivity or the aperture value appears across the resistor R1, and thus, the logarithmically compressed information of film sensitivity or aperture value appears in the form of voltage Vr across the resistor R1. This logarithmically compressed information of film sensitivity and aperture value represent APEX indices Sv and Av, respectively. On the other hand, between the collector of the transistor Q1 and ground there is, as mentioned above, the voltage V1 which is equal to the voltage Vs that is proportional to the luminance value Bv. Therefore, across the diode D1 there appears a voltage Vd which is the difference between the voltage V1 and voltage Vr, expressed by the equation Vd=V1-Vr.
In the above equation, the voltage V1 is proportional to the luminance value Bv and the voltage Vr is proportional to the difference between the aperture value Av and the film sensitivity value Sv. Therefore, the relation of Vd.alpha.Bv-(Av-Sv) holds, and by the known equation Tv=Bv-(Av-Sv) for providing an exposure-time value Tv, the voltage Vd provides the exposure-time value Tv. By this value Tv, the exposure time is controlled, but actually, a logarithmic expansion of the voltage Vd for providing the exposure-time value is used. In other words, a current proportional to an inverse logarithm of the voltage Vd flows in the diode D1, and this current is fed to an integrating capacitor to convert it into voltage. The exposure time is controlled by the voltage across this capacitor.
Thus, in the above-mentioned circuit of FIG. 1 showing the prior art, a variable resistor for exponentially changing the resistance value is necessary. Moreover, for the purpose of producing a voltage precisely proportional to the wide-range information Sv and Av of the film sensitivity and the aperture value, respectively, across the resistor R1, the voltage to be impressed on the base of the transistor Q2 must be comparatively high in view of the characteristic of the emitter-follower circuit. For filling these requisites, such expedients as increasing the number of diodes corresponding to diode D2, increasing the power-source voltage, etc. have been employed at an expense of a greater power consumption for the circuit and other disadvantages.